The present invention relates to a measuring sensor and a method for fabricating the measuring sensor.
German Published Patent Application No. 197 07 458 describes a measuring sensor. This document further describes a joining method for the measuring sensor. In the measuring sensor, a molded ceramic housing includes two parts, namely, a ceramic molding on the measuring-gas side and a ceramic molding on the connector side, which are arranged lying axially one behind the other. Between the two ceramic moldings, a cavity is formed in which a warm-deformable metallic seal element is inserted. The two parts of the ceramic housing serve as compressing elements in the insertion operation. The seal element in the measuring sensor can include a ductile metal body made of copper, aluminum, palladium, nickel, silver, or an alloy of these metals with additives such as graphite, boron nitride, talc, bentonite, kaolin, or the metal body is a composite body having a core with a surface coating on one side or both sides. Alternatively, the seal element can also be a metal solder into which additives such as graphite, boron nitride, talc or a mixture of these additives are admixed.
It is an object of the present invention to provide a measuring sensor and a method for its fabrication such that no crack-initiating stresses arise in the component upon the occurrence of temperature gradients or of temperature intervals.
Reliable hermetic sealing of the inserted sensor element against gas and gasoline is achieved according to the present invention because the molded housing is designed in one piece and exhibits an axially symmetric and cylindrically shaped longitudinal hole on the connector side, around the sensor element, and/or a metal shell pushed onto the molded housing in gastight fashion on the connector side, and the seal element is inserted in the longitudinal hole and/or metal shell between a first compressing element on the measuring-gas side and a second compressing element on the connector side such that it tightly encloses the sensor element in the molded housing or, respectively, in the metal shell and hermetically seals the sensor element with respect to the molded housing and/or the metal shell. Damage to the molded ceramic housing and cracking of the molded ceramic housing upon insertion are averted because, for example, the molded ceramic housing no longer serves as a compressing element.
Thus, according to the present invention, the sensor element is inserted into a molded ceramic housing or into a metal shell pushed onto a ceramic molding to construct the joint, subject to the conditions of the joining operation.
The fabrication method according to the present invention, with which a measuring sensor according to the present invention can be fabricated, includes the following steps:
A. The first compressing element is inserted into the longitudinal hole of the molded ceramic housing or into the metal shell with inserted sensor element;
B. The seal element is laid on first compressing element and around the sensor element; the second compressing element is arranged (laid) on top of it;
C. The workpiece, preassembled to this point, is placed in a temperature-resistant workpiece carrier;
D. The workpiece carrier with inserted workpiece is heated in a furnace, preferably in air, to a particular temperature for a particular length of time;
E. The workpiece carrier with inserted workpiece is removed from the furnace;
F. With an external ram, the second compressing element is inserted warm, at a certain temperature, preferably in air, onto and into the seal element inside the longitudinal hole of the molded housing or inside the metal shell, until the ram makes contact with the molded housing or the metal shell.
A reliable and economical method for the hermetic sealing of sensor elements in the sensor housing is provided by the fabrication method according to the present invention. Warm insertion makes possible the use of stable glasses with relatively high transformation temperature at a low joining temperature, which results in, for example, no excessive thermal stress for the sensor element. Further, metal-containing seal materials can be used, which have significant advantages over pure glass castings.
The first compressing element is preferably a powder compact made of temperature-resistant material, preferably with good frictional qualities, such as, for example, steatite, talc, graphite, boron nitride, or copper. Alternatively, the first compressing element is made of glass, glass-ceramic, oxide-ceramic with the addition, as appropriate, of organic binders, placed as loose-poured powder or granules, as a compact, presintered, as appropriate, and/or as a cast film; the first compressing element is preferably inserted cold to fix the sensor element in place.
The seal element is made of warm-deformable, temperature-resistant material, preferably of glass and/or metal, advantageously copper, with the addition, as appropriate, of organic binder and other reducing agents, placed as loose-poured powder, granules or compound (xe2x80x9cpanatxe2x80x9d), as a compact, presintered as appropriate, and/or as a cast film.
The second compressing element is made of ceramic powder, high-melting glass powder and/or metal powder, alternatively of steatite or talc, which are preferably not presintered and to which organic binding agents and or compacting aids are added, or, further alternatively, of zirconium oxide or, respectively, Baxe2x80x94Al silicate glass, which is preferably sintered. The end face of the second compressing element may have projections, protruding toward the seal element, with which the second compressing element can be anchored in the seal element.
In the joining operation (steps A-F), pores in the seal element are eliminated or open pores are closed. As the temperature is raised during the joining operation, non-presintered compressing elements are presintered to the extent that they acquire sufficient strength for the compression of the seal element.
The external ram is preferably split, and a section of the external ram set on the part being joined, which section is preferably made of sintered Al2O3 or ZrO2 ceramic, is co-heated to the joining temperature. Then the external ram is preferably pushed until stopped by the molded ceramic housing or the metal shell, so that the second compressing element makes flush contact with the connector-side end surface of the molded ceramic housing or of the metal shell.